In general, on a finish rolling machine inlet side of a line for manufacturing a hot-rolled steel sheet (hereinafter referred to also as a hot strip mill), there is performed cutting of unsteady deformed portions called crop portions formed at a front end and a rear end of a steel sheet (a sheet bar or an intermediate material) in a steel sheet conveying direction (which is also a rolling direction) in a rough rolling step to stabilize steel sheet conveyance (also referred to as sheet passing) during finish rolling. The front end of the steel sheet in the conveying direction and the rear end thereof in the conveying direction are deformed into various shapes by width reduction of a sizing press, width rolling of a width rolling machine, horizontal rolling of a rough rolling machine and the like. FIGS. 1A to 1D illustrate examples of an outline (a planar shape) of the crop portion when each of the front end and the rear end of the steel sheet in the conveying direction are seen from above. A shape of FIG. 1A is called a fishtail and a shape of FIG. 1B is called a tongue. In the tongue shape, a central portion of the steel sheet in a width direction projects in the conveying direction to both ends of the steel sheet in the width direction. In the fishtail shape, both the ends of the steel sheet in the width direction project in the conveying direction to the central portion thereof in the width direction.
In the present description, a portion A illustrated in FIG. 1A is a concave portion bottom of the fishtail shape and a portion B is a convex portion top of the fishtail shape. Furthermore, a length from the concave portion bottom (the portion A) to the convex portion top (the portion B) of the fishtail shape is also referred to as a fishtail length. Furthermore, in a case where the fishtail shape is asymmetrical and right and left fishtails have different lengths, the smaller length of the two lengths is defined as the fishtail length. Depending on conditions of the rough rolling step, the planar shape of the crop portion also becomes such an asymmetric shape as illustrated in FIG. 1C or FIG. 1D in which both sides of the steel sheet in the width direction are asymmetric to the center thereof in the width direction. When this crop portion of the asymmetrical shape in the width direction is formed at the front end or the rear end of the steel sheet in the conveying direction and the steel sheet is passed through the finish rolling machine, an unbalanced load would be generated in a finish rolling roller in the steel sheet width direction, and as a result, there is the possibility that the steel sheet meanders during the finish rolling.
Furthermore, at the front end of the steel sheet in the conveying direction, heat is released from four surfaces of a most distal surface in the conveying direction, an end surface in the width direction, an upper surface and a lower surface, a temperature of the front end therefore noticeably drops, and the temperature becomes lower than that of a steady portion. This front end of the steel sheet in the conveying direction at which the temperature is low has a large deformation resistance, and therefore becomes a cause for a biting defect in the finish rolling machine. Furthermore, at the rear end of the steel sheet in the conveying direction, heat is released from four surfaces of a most proximal surface in the conveying direction, the end surface in the width direction, the upper surface and the lower surface, a temperature of the rear end therefore noticeably drops, and the temperature becomes lower than that of the steady portion. This rear end of the steel sheet in the conveying direction at which the temperature is low also has a large deformation resistance, and during the finish rolling, squeezing of the steel sheet easily occurs. As described above, after end of the rough rolling step and before the finish rolling, there is performed cutting of the crop portions of the front end of the steel sheet in the conveying direction and the rear end thereof in the conveying direction. The cutting of the crop portions are performed with a crop cutting machine (also referred to as a crop shear). When the crop portions of the front end of the steel sheet in the conveying direction and the rear end thereof in the conveying direction are cut, there can be obtained effects such as prevention of the meandering of the steel sheet in the finish rolling, stabilization of the biting of the steel sheet and prevention of the squeezing of the steel sheet.
Furthermore, in the cutting of the crop portions of the front end of the steel sheet in the conveying direction and the rear end thereof in the conveying direction, a cutting load noticeably varies with a type of hot-rolled steel sheet (product) to be manufactured. In recent years, an increasingly demanded steel sheet for a line pipe material has also been manufactured in the line for manufacturing the hot-rolled steel sheet (the hot strip mill). In this manufacturing of the hot-rolled steel sheet for the line pipe material, the cutting load of the crop cutting machine in the cutting of the crop portions of the front end of the steel sheet in the conveying direction and the rear end thereof in the conveying direction remarkably increases as compared with conventional manufacturing of a usual hot-rolled steel sheet. For the line pipe material, there are required specifications of high strength and extra thickness from the viewpoint of highly efficient transport of crude oil, natural gas or the like. Furthermore, a pipeline is also laid in a seismic zone, and hence high tenacity is also required for the line pipe material. Consequently, in the case where the hot-rolled steel sheet for the line pipe material is manufactured in the line for manufacturing the hot-rolled steel sheet (the hot strip mill), there are points to consider as follows.
The first point is a sheet thickness of the steel sheet. In the conventional hot-rolled steel sheet in which the thickness of the finish-rolled sheet is from about 2 to 4 mm, the sheet thickness of the intermediate material (the sheet bar) prior to the finish rolling is from 30 to 50 mm. On the other hand, in the case of the hot-rolled steel sheet for the line pipe material for which the high tenacity is required, control rolling called TMCP (thermo-mechanical control process) is performed to miniaturize a crystal structure, thereby acquiring the tenacity of the steel sheet, and in this case, it is necessary to increase a reduction ratio in the finish rolling. A product sheet thickness required in the hot-rolled steel sheet for the line pipe material is 20 mm or more and 30 mm or less, and further, for the purpose of obtaining the tenacity required for the line pipe material, a cumulative reduction ratio in the finish rolling needs to be at least 60%. That is, to manufacture the hot-rolled steel sheet for the line pipe material which has a sheet thickness of 20 mm in the line for manufacturing the hot-rolled steel sheet, the steel sheet of the intermediate material having a sheet thickness of 50 mm or more has to be finish-rolled. However, in the existing line for manufacturing the hot-rolled steel sheet, there is assumed the intermediate material of the usual steel sheet in which the sheet thickness of is from 30 to 50 mm, and a cutting load upper limit value of the crop cutting machine is specified in conformity to the conventional intermediate material sheet thickness. Therefore, to manufacture the hot-rolled steel sheet for the line pipe material in the existing line for manufacturing the hot-rolled steel sheet, there is required a technology of cutting the crop portion of the intermediate material having a sheet thickness of 50 mm or more with the existing crop cutting machine.
The second point to consider which is important in manufacturing the hot-rolled steel sheet for the line pipe material in the line for manufacturing the hot-rolled steel sheet (the hot strip mill) is a sheet width of the steel sheet (the intermediate material). There is also a case where the line pipe material is manufactured as a spiral steel tube. In this case, decreasing of welded portions of the steel tube as much as possible is more advantageous in terms of strength, and hence the hot-rolled steel sheet having a larger width is required as the pipe material. In general, the sheet width required as the hot-rolled steel sheet for the line pipe material is 1200 mm or more and 2100 mm or less, and it is necessary to cut the crop portion of the intermediate material having a sheet width of 1200 mm or more with the crop cutting machine.
The third point to consider which is important in manufacturing the hot-rolled steel sheet for the line pipe material in the line for manufacturing the hot-rolled steel sheet (the hot strip mill) is a temperature of the steel sheet (the intermediate material). For the purpose of obtaining the hot-rolled steel sheet having the high tenacity, the finish rolling has to be performed at a temperature of a non-recrystallizing region. Consequently, it is necessary to perform finish rolling by setting a sheet thickness center temperature of the steel sheet to 930° C. or less from the front end in the conveying direction to the rear end in the conveying direction. Therefore, when the crop portions of the front end of the steel sheet in the conveying direction and the rear end thereof in the conveying direction are cut, the temperature of each crop portion at a cutting location is also in the vicinity of 930° C. and becomes lower than that a temperature (about 1000° C.) of the usual steel sheet. Therefore, in the hot-rolled steel sheet for the line pipe material (the intermediate material), a cutting resistance value is higher and the cutting load increases as compared with the conventional steel sheet.
When the above-mentioned points to consider are taken together, the hot-rolled steel sheet for the line pipe material manufactured in the line for manufacturing the hot-rolled steel sheet (the hot strip mill) has a larger sheet thickness, a larger sheet width and a lower temperature, and in the crop cutting machine, the cutting load larger than that of the conventional hot-rolled steel sheet is applied to the hot-rolled steel sheet. Furthermore, the specifications of the existing crop cutting machine are designed in accordance with the heretofore manufactured conventional hot-rolled steel sheet, and hence for the purpose of manufacturing the hot-rolled steel sheet for the line pipe material without any noticeable equipment modification such as reinforcement of the crop cutting machine, a technique of cutting the steel sheet into a sheet thickness of 50 mm or more and a sheet width of 1200 mm or more and at a temperature of 930° C. or less is required also in the specifications of the existing crop cutting machine.
Concerning the cutting of the crop portions of the front end of the steel sheet in the conveying direction and the rear end thereof in the conveying direction on the inlet side of the finish rolling machine, various technologies have been suggested from the viewpoints of sheet passing properties in the finish rolling machine and decrease of yield loss. For example, in Patent Literature 1 mentioned below, there has been suggested a method of predicting a shape of the crop portion of the finish-rolled steel sheet from a shape of the crop portion of each of the front end of the steel sheet (the intermediate material) in the conveying direction and the rear end thereof in the conveying direction, evaluating an appearance of the steel sheet as the product, judging presence/absence of the crop portion to be cut, and automatically adjusting a cutting length. In this method, there is included a case where the crop portion of the fishtail shape is cut. Further, according to this method, the steel sheet can be rolled without cutting the crop portions of the front end of the steel sheet in the conveying direction and the rear end thereof in the conveying direction, or even when the crop portions are cut, the cutting length may be a minimum length, and hence the yield improves. Furthermore, for example, in Patent Literature 2 mentioned below, there has been suggested a method of measuring, with a shape meter, the shape of each of the crop portions of the front end of the steel sheet (the intermediate material) before cut in the conveying direction and the rear end thereof in the conveying direction, determining an optimum cutting length from the measured shape in consideration of the biting defect of the finish rolling machine, a quality and the yield, and cutting each crop portion into the cutting length. According to this method, sheet passing troubles decrease and the quality and yield improve.